A blended source drawing from wells No.1 and No.2, a sodium-led bicarbonate water with a chloride sub-type — the classic sodium bicarbonate (jūsō-sen) style finished with a salty edge. Both wells emerge in the low-50s °C and are diluted only when the season pushes them too hot, so the bath stays weakly alkaline and soft on the skin, sitting squarely in Japan's beauty-water family. Metasilicic acid at 62 mg/kg leaves a thin silica film that smooths the skin, while a faint hydrogen sulfide note trails a mildly salty taste.
Good for
Smooth skin
Bicarbonate and alkalinity gently lift dead keratin while silica hydrates, leaving skin smooth. This is the basis of the beautifying-water (bijin-no-yu 美人の湯) reputation.
dry, rough skin
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Full composition
The bar is each ion's share of charge (mval%) within its group; over 20% names the spring. Gases, silica and trace metals are measured by mass only.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺498mgP74 national80%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺66.8mgP62 national12%
- Magnesiummild; a laxative if you drink itMg²⁺16.4mgP68 national5%
- Potassiumminor; matters for drinking limitsK⁺21.6mgP65 national2%
- LithiumLi⁺1.0mgP73 national0.5%
- StrontiumSr²⁺1.9mgP82 national0.2%
- AmmoniumNH₄⁺0.70mgP52 national0.1%
- BariumBa²⁺1.9mgP93 national0.1%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,046mgP90 national65%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻318mgP65 national34%
- FluorideF⁻4.4mgP77 national0.9%
- SulfateSO₄²⁻8.1mgP22 national0.6%
- Hydrogen phosphateHPO₄²⁻0.70mgP75 national0.1%
- Hydrogen sulfideHS⁻0.40mgP50 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃62.3mgP40 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂19.0mgP71 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂42.2mgP65 national
- ArsenicAs< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn—mg
Measured at the source
Evidence
- Maeda M. 温泉の医学的効果とその科学的根拠. J. Hot Spring Science 70:197–207, 2021.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.